The overall objective continues to be the development and perfection of isotope techniques to produce a clinically applicable diagnostic method for deep vein thrombosis; also, to increase the diagnostic accuracy of impedance plethysmography and Doppler ultrasound as non-invasive diagnostic procedures. The goals of this past year were accomplished through the close collaboration of Hematologyy, Nuclear Medicine, Vascular Surgery and Blood Flow Laboratory -in the labelling of white blood cells with Technetium-99m sulfur colloid by phagocytosis; the administration and localization of labelled white blood cells in animals with propagating deep vein thrombi; the localization of fluorescent labelled microspheres in performed thrombi in vivo; the study of the pharmacokinetics in animals of labelled urokinase, streptokinase and fribrinogen to develop a radiation dosimetry model prior to clinical trials; the use of a tilting table in impedance plethysmography to study venous return; to study the effect of exercise on venous compliance; the analysis of patients entered into the blood flow diagnostic program to determine the accuracy of combined non-invasive diagnostic methods by IPG and Doppler ultrasound, including the incidence of "silent embolization" to the lung, "silent leg" in pulmonary embolism as the initial symptom, pulmonary embolism as related to anatomical site of venous thrombosis. Work for the next year will be directed towards documentation of a new model for deep vein thrombosis in experimental animals, and to continue the labeling of white blood cells. With successful results in animal studies, a strong consideration will be given to clinical trials of autologous labelled leukocytes for the diagnosis of deep vein thrombi; and, although radionuclide venography has shown very promising clinical results in identifying deep venous thrombosis, a number of questions concerning its use remain unanswered, necessitating investigation in an appropriate animal system.